diff --git a/pkg/runtime/compactor/compactor.go b/pkg/runtime/compactor/compactor.go
index e4addce95..721edd2d7 100644
--- a/pkg/runtime/compactor/compactor.go
+++ b/pkg/runtime/compactor/compactor.go
@@ -122,7 +122,7 @@ func RunLLM(ctx context.Context, args LLMArgs) (*Result, error) {
 		options.WithStructuredOutput(nil),
 		options.WithMaxTokens(MaxSummaryTokens),
 	)
-	compactionAgent := agent.New("root", compaction.SystemPrompt, agent.WithModel(summaryModel))
+	compactionAgent := agent.New("root", "", agent.WithModel(summaryModel))
 
 	messages, firstKeptEntry := extractMessages(args.Session, compactionAgent, args.ContextLimit, args.AdditionalPrompt)
 
diff --git a/pkg/runtime/compactor/compactor_test.go b/pkg/runtime/compactor/compactor_test.go
index 301dfaa0f..bfd638415 100644
--- a/pkg/runtime/compactor/compactor_test.go
+++ b/pkg/runtime/compactor/compactor_test.go
@@ -12,9 +12,25 @@ import (
 	"github.com/docker/docker-agent/pkg/agent"
 	"github.com/docker/docker-agent/pkg/chat"
 	"github.com/docker/docker-agent/pkg/compaction"
+	"github.com/docker/docker-agent/pkg/model/provider/base"
 	"github.com/docker/docker-agent/pkg/session"
+	"github.com/docker/docker-agent/pkg/tools"
 )
 
+type fakeProvider struct{ id string }
+
+func (p fakeProvider) ID() string { return p.id }
+
+func (p fakeProvider) BaseConfig() base.Config { return base.Config{} }
+
+func (p fakeProvider) CreateChatCompletionStream(
+	context.Context,
+	[]chat.Message,
+	[]tools.Tool,
+) (chat.MessageStream, error) {
+	return nil, nil
+}
+
 func TestExtractMessages(t *testing.T) {
 	t.Parallel()
 
@@ -202,6 +218,38 @@ func TestMapToSessionIndex(t *testing.T) {
 	assert.Equal(t, len(sess.Messages), mapToSessionIndex(sess, 3))
 }
 
+func TestRunLLM_DoesNotDuplicateSystemPrompt(t *testing.T) {
+	t.Parallel()
+
+	sess := session.New(session.WithMessages([]session.Item{
+		session.NewMessageItem(&session.Message{Message: chat.Message{Role: chat.MessageRoleUser, Content: "please summarize"}}),
+	}))
+	a := agent.New("test", "parent prompt", agent.WithModel(fakeProvider{id: "fake/model"}))
+
+	var systemPromptCount int
+	result, err := RunLLM(t.Context(), LLMArgs{
+		Session:      sess,
+		Agent:        a,
+		ContextLimit: 100_000,
+		RunAgent: func(_ context.Context, compactionAgent *agent.Agent, compactionSession *session.Session) error {
+			for _, msg := range compactionSession.GetMessages(compactionAgent) {
+				if msg.Role == chat.MessageRoleSystem && msg.Content == compaction.SystemPrompt {
+					systemPromptCount++
+				}
+			}
+			compactionSession.AddMessage(&session.Message{Message: chat.Message{
+				Role:    chat.MessageRoleAssistant,
+				Content: "summary",
+			}})
+			return nil
+		},
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	assert.Equal(t, 1, systemPromptCount, "compaction sub-run should see the compaction system prompt exactly once")
+}
+
 // TestRunLLM_RequiresRunAgent pins the contract that a missing RunAgent
 // callback is rejected loudly rather than silently no-oping.
 func TestRunLLM_RequiresRunAgent(t *testing.T) {